1. Field of the Invention
The present invention relates to a wire bonding apparatus and more particularly to a wire bonding apparatus that forms a ball at a tip end of a bonding wire by an electric discharge caused by applying a high voltage across the tip end of the wire and a discharge electrode.
2. Prior Art
A wire bonding apparatus is equipped with a capillary that allows a wire to pass therethrough and the tip end of the wire to protrude therefrom and a discharge electrode that is installed separately from the capillary. A spark is generated between the electrode and the tip end of the wire, so that the tip end of the wire is melted by the spark, thus forming a ball at this tip end, and the ball is bonded to an object such as an electrode, etc.
Apparatuses of this type that utilize a discharge include fixed discharge electrode systems and movable discharge electrode systems. In the fixed discharge electrode systems, the discharge electrode is fixed; and in the movable discharge electrode systems, the discharge electrode is movable by means of a solenoid, etc. An example of the fixed discharge electrode system is described in Japanese Patent Application Laid-Open (Kokai) No. 7-263480. Examples of movable discharge electrode systems are described in Japanese Patent Application Laid-Open (Kokai) Nos. 5-36748, 5-102233 and 7-176560.
In the fixed discharge electrode system, a discharge electrode is disposed so as to be on one side of the tip end of the wire protruding from the capillary. In other words, the discharge electrode is positioned as close as possible to the tip end of the wire, and there is no contact of the electrode with the capillary when the capillary is lowered. In the movable discharge electrode system, the discharge electrode is positioned directly beneath the tip end of the wire; and the discharge electrode is retracted from beneath the capillary after a discharge is performed and the ball is formed.
The fixed discharge electrode system requires, unlike the movable discharge electrode system, no driving mechanism and control mechanism that drives the discharge. Accordingly, such an apparatus has superior features. The weight of the bonding head as a whole is reduced, and an increase in operation speed is facilitated. Furthermore, the cost of the apparatus is less, and maintenance can easily be performed.
However, in this fixed discharge electrode system, as seen from FIG. 4, the discharge electrode 103 is disposed on one side of the tip end of wire 102 that passes through the capillary 101. Thus, when the discharge is performed, the spark 108a, 108b or 108c is discharged toward the wire 102 from one side of the wire 102. As a result, the tip end of the wire 102 is melted from the direction that faces the spark 106a, 106b or 106c, thus causing balls 107a, 107b and 107c to be gradually formed, and as a result, a ball 107d that is distorted or eccentric from the axial center of the wire 102 as shown in FIG. 4B is formed.
If contaminants such as ions, etc. adhere to the tip end portion of the discharge electrode 103, the discharge will occur from the side surface of the discharge electrode 103 as indicated by the spark 108c of FIG. 4A. In this case, the wire 102 begins to melt from the position of the ball 107c; as a result, the wire 102 is cut at an intermediate point, so that no ball is formed or an extremely small ball is formed. If the wire 102 is not cut at an intermediate point, a ball 107e with a distorted shape as shown in FIG. 4C is formed that will cause variation in the ball diameter.
Accordingly, the object of the present invention is to provide a bonding apparatus that assures a stable ball formation by way of correcting the path of the spark and prevents adhesion of contaminants to the discharge electrode.
The above object is accomplished by a unique structure of the present invention for a wire bonding apparatus that includes: a capillary, which allows a tip end of a wire to protrude therefrom, and a discharge electrode, which is disposed separately from the capillary and generates a spark between the electrode and the tip end of the wire by an electrical discharge, thus melting the tip end of the wire by the spark and forming a ball that is to be bonded to an object of bonding; and in the present invention, the wire bonding apparatus further includes auxiliary electrodes which generate an electric field that acts on a specified region between the tip end of the wire and a tip end of the discharge electrode, thus correcting the path of the spark by the electric field.
In this structure, the path of the spark that is generated between the discharge electrode and the tip end of the wire that protrudes from the capillary is corrected by the electric field generated by the auxiliary electrodes. Accordingly, incomplete ball formation that would be caused by the deviation of the spark path is prevented. Furthermore, the adhesion of contaminants to the discharge electrode is suppressed since foreign matters are caught by the auxiliary electrodes.
In the above structure of the wire bonding apparatus, the discharge electrode is disposed outside the moving path of the capillary, and the above-described advantages are further enhanced.
Furthermore, in the present invention, the end portion of the spark on the wire side can be guided onto the axial line of a portion of the wire that protrudes from the capillary by the electric field that is generated by the auxiliary electrodes. Accordingly, incomplete ball formation caused by deviation of the end portion of the spark on the wire side can be prevented.
Further, it can be designed so that the end portion of the spark on the discharge electrode side is guided onto the axial line of the discharge electrode by the electric field that is generated by the auxiliary electrodes. With this structure, incomplete ball formation caused by the deviation of the end portion of the spark on the discharge electrode side can be prevented.
In addition, in the bonding apparatus of the present invention, the auxiliary electrodes have the same polarity as that of the tip end of the wire or the discharge electrode. With the auxiliary electrodes having the same polarity as the tip end of the wire or the discharge electrode, the position of the end portion of the spark in the vicinity of the tip end of the wire or discharge electrode can be attracted toward the auxiliary electrodes.
Instead, the auxiliary electrodes can have a polarity that differs from the polarity of the tip end of the wire or the discharge electrode. With the auxiliary electrodes that have a different polarity than that of the tip end of the wire or the discharge electrode, the position of the end portion of the spark in the vicinity of the tip end of the wire or discharge electrode can be repelled toward the opposite side from the auxiliary electrodes.